1. Field of the Invention
The invention relates to a tunable optical system with hybrid integrated semiconductor laser. In particular, the invention relates to a hybrid integrated tunable laser module using silicon external cavity and silicon waveguide wavelength tuning structures.
2. Description of the Related Art
The ever increasing bandwidth demand resulting from the continuous pursuit of high bandwidth data communication, especially the recent effort in wavelength-division multiplexing (WDM) passive optical network (PON), has motivated the fiber optic communication technology into a new era. The next generation optic communication system has to have the capability of handling much higher data communication traffic. In addition to the performance requirement, low cost solution is highly desired to reduce the total network construction and reconfiguration cost. A direct way to extend the already saturated capacity of current fiber optic communication systems is to transmit multiple optical channels using one single channel fiber using the dense WDM (DWDM) technology.
A WDM optical communication system, particularly a WDM-PON, typically consists of WDM optical transmitters, receivers, and WDM filters. The WDM receiver includes multiple photodetectors (PDs) and a WDM de-multiplexing (DMUX) filter. Since PDs can operate in a wide spectrum range, no particular requirement is imposed on them when used in WDM systems. The WDM optical transmitter includes a series of light sources emitting at multiple wavelengths and a WDM multiplexing (MUX) filter. The commonly used MUX/DMUX filters include diffraction gratings, thin-film filters, arrayed waveguide gratings, and optical add/drop filters, etc. The key and most challenging component is the light source module, which employs a series of light sources operating at different wavelengths. In most cases, they are strictly required to operate at ITU (International Telecommunication Union) frequency grids. To manufacture, deploy and manage these lights sources with so many different operating wavelengths is very challenging and cost ineffective.
A low cost tunable laser module with a wide range of wavelength reconfigurability is of particular importance for WDM-PON systems. Tunable lasers widely used in traditional long-haul WDM optical communication systems include distributed feedback (DFB) lasers, sampled-grating distributed Bragg reflector (SG-DBR) lasers, and external cavity lasers (ECLs). DFB lasers usually tune wavelength through thermal effect. Their wavelength tuning range is limited to less than a few nanometers. SG-DBR lasers monolithically integrate multiple active/tuning sections in one III-V material based chip and tune wavelength through current injection. They use Vernier effect to expand the tunability of the laser and can have much wider wavelength tuning range. However, SG-DBR lasers may not be a suitable light source solution for WDM-PON applications due to high cost and complex process. ECLs have many variations depending on the chosen type of external cavities. The approaches to tune the lasing wavelength of an ECL include changing laser resonant cavity and using wavelength tuning mirrors. Direct change of laser resonant cavity sometimes is difficult and may introduce additional fabrication complexity and high cost. Wavelength tuning mirrors are commonly used in ECLs. However, in most cases, integrating these tunable mirrors with laser active chips and executing the tuning are very challenge tasks. The ECLs will be a suitable light source for WDM-PON applications if low cost solution of fabricating, packaging and tuning the external cavity can be achieved.